We didn't know if there was impact debris, if there were things accreting on these objects; something the size of a grain of rice would knock us off course.
When there was impact between groups, complex models had lower bias under certain conditions, but were not as efficient as simple models in estimating DIF parameters.
HLR accounting for the person clustering effect can control type I error rate as well as the other methods when there was no impact between groups, but exhibited serious type I error inflation when there was impact between groups.
Korea (average math achievement score of 613 with SD of 2.9) and New Zealand (average math achievement score of 488 with SD of 5.5) were selected to reflect the condition when there was impact between countries.
When there was impact between groups, the multilevel testlet model was most accurate in terms of estimating DIF parameter among the four methods when the studied item's difficulty parameter was low or medium, but was the least efficient method due to its complexity.
As secondary objectives, we attempted to identify risk factors for such events and tools that could detect them early as well as determine if there was impact on LOS.
